Toward the Sustainable Synthesis of Propanols from Renewable Glycerol Over Moo3-Al2o3 Supported Palladium Catalysts

catalysts

Article Toward the Sustainable Synthesis of Propanols from Renewable Glycerol over MoO3-Al2O3 Supported Palladium Catalysts

Shanthi Priya Samudrala * and Sankar Bhattacharya

Department of Chemical Engineering, Faculty of Engineering, Monash University, Melbourne 3800, Australia; [email protected] * Correspondence: [email protected]; Tel.: +61-3-9905-8162

Received: 17 August 2018; Accepted: 6 September 2018; Published: 9 September 2018

Abstract: The catalytic conversion of glycerol to value-added propanols is a promising synthetic route that holds the potential to overcome the glycerol oversupply from the biodiesel industry. In this study, selective hydrogenolysis of 10 wt% aqueous bio-glycerol to 1-propanol and 2-propanol was performed in the vapor phase, ﬁxed-bed reactor by using environmentally friendly bifunctional Pd/MoO3-Al2O3 catalysts prepared by wetness impregnation method. The physicochemical properties of these catalysts were derived from various techniques such as X-ray diffraction, 27 NH3-temperature programmed desorption, scanning electron microscopy, Al NMR spectroscopy, surface area analysis, and thermogravimetric analysis. The catalytic activity results depicted that a high catalytic activity (>80%) with very high selectivity (>90%) to 1-propanol and 2-propanol was obtained over all the catalysts evaluated in a continuously fed, ﬁxed-bed reactor. However, among all others, 2 wt% Pd/MoO3-Al2O3 catalyst was the most active and selective to propanols. The synergic interaction between the palladium and MoO3 on Al2O3 support and high strength weak to moderate acid sites of the catalyst were solely responsible for the high catalytic activity. The maximum glycerol conversion of 88.4% with 91.3% selectivity to propanols was achieved at an optimum reaction condition of 210 ◦C and 1 bar pressure after 3 h of glycerol hydrogenolysis reaction.

Keywords: hydrogenolysis; glycerol; 1-propanol; 2-propanol; palladium catalyst

1. Introduction Alternate sustainable energy resources are vital because of dwindling petroleum reserves and mounting environmental alarms that are allied with fossil fuel exploitation. As a result, alternative bio-based fuels have emerged as the long-standing solution as they are renewable and carbon dioxide neutral [1]. Biodiesel is one such alternative fuel which has received much attention with both demand and production tremendously increased over the last few years. However, along with biodiesel, also known as alkyl esters of long chain fatty acids (C14–C24), a great deal of glycerol as a by-product is also generated, typically equaling 10% of the whole production volume. The crude glycerol, if not handled properly, ends up as a waste product that has low value and is costly to purify in addition to jeopardizing the environmentally friendly nature of the whole biodiesel production process [2]. Valorization of glycerol is, therefore, necessary to enhance the sustainability of the biodiesel industry. Glycerol, the simplest tri-hydroxy alcohol, is a highly versatile product, with many potential applications. The biodegradability and multi-functional nature of glycerol makes it a promising precursor for the production of high-value bio-renewable fuel/chemical products through various processes involving heterogeneous catalysis, e.g., acetalization [3,4], esteriﬁcation [5], etheriﬁcation [6], oxidation [7], dehydration [8], hydrogenolysis [9], and catalytic reforming [10]. The glycerol-derived

Catalysts 2018, 8, 385; doi:10.3390/catal8090385 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 385 2 of 18

Catalysts 2018, 8, x FOR PEER REVIEW 2 of 18 fuelprocesses and chemical involving products heterogeneous include catalysis, liquid/gaseous e.g., acetalization fuels, [3,4], fuel esterification additives, and[5], etherification chemicals such as solketal,[6], oxidation glycerol [7], mono-esters,dehydration [8], glyceric hydrogenolysis acid, 1,3-dihydroxyacetone, [9], and catalytic reforming epichlorohydrin, [10]. The glycerol- glycidol, tartronicderived acid, fuel lacticand chemical acid, acrylonitrile,products include 1,2-propanediol, liquid/gaseous fuels, and fuel 1,3-propanediol, additives, and chemicals etc. such Catalytic hydrogenolysisas solketal, glycerol of glycerol mono-esters, is a promising glyceric acid, approach, 1,3-dihydroxyacetone, resulting in the epichlorohydrin, formation of glycidol, commodity chemicalstartronic such acid, as 1,2-propanediol, lactic acid, acrylonitrile, 1,3-propanediol, 1,2-propanediol, ethylene and glycol, 1,3-propanediol, propanols etc. (1-propanol Catalytic and 2-propanol),hydrogenolysis lower alcohols, of glycerol and is ahydrocarbons promising approach, [11]. Signiﬁcant resulting inefforts the formation have been of made commodity to convert glycerolchemicals into propanediols, such as 1,2-propanediol, but direct 1,3-propanediol, production of propanolsethylene glycol, via glycerol propanols hydrogenolysis (1-propanol and received 2- propanol), lower alcohols, and hydrocarbons [11]. Significant efforts have been made to convert limited attention. glycerol into propanediols, but direct production of propanols via glycerol hydrogenolysis received 1-propanol (1-PrOH) and 2-propanol (2-PrOH) are valuable commodity chemicals conventionally limited attention. produced1-propanol from fossil-based (1-PrOH) feedstocks and 2-propanol ethylene and (2-PrOH) propylene are by valuable hydroformylation-hydrogenation commodity chemicals and hydrationconventionally reaction produced processes, from fossil-based respectively feedstocks [12]. 1-PrOH ethylene has and potential propylene applications, by hydroformylation- primarily as a solventhydrogenation in the pharmaceutical, and hydration paint, reaction cosmetics, processes, and cellulose respectively ester industries, [12]. 1-PrOH organic has intermediate potential for the synthesisapplications, of important primarily chemical as a solvent commodities, in the pharmaceutical, and considered paint, as cosmetics, the next-generation and cellulose gasoline ester to petroleumindustries, substitute. organic 2-PrOHintermediate is extensively for the synthesis used of as important an industrial chemical solvent commodities, and disinfectant, and considered with major applicationsas the next-generation in the pharmaceutical gasoline to industrypetroleum and substitute. auto industry 2-PrOH is [13 extensively]. Due to theused growing as an industrial demand of thesesolvent commodities, and disinfectant, the production with major of applications 1-PrOH and in 2-PrOHthe pharmaceutical based on bio-basedindustry and glycerol auto industry (Scheme 1) [13]. Due to the growing demand of these commodities, the production of 1-PrOH and 2-PrOH based appears to be an attractive approach regarding sustainability and energy efﬁciency compared to the on bio-based glycerol (Scheme 1) appears to be an attractive approach regarding sustainability and process based on petroleum-derived feedstocks. energy efficiency compared to the process based on petroleum-derived feedstocks.

SchemeScheme 1. 1.Synthetic Syntheticroutes routes to 1-propanol and and 2-propanol. 2-propanol.

HydrogenolysisHydrogenolysis of glycerolof glycerol generally generally takestakes place via via the the dehydration-hydrogenation dehydration-hydrogenation pathway pathway overover metal-acid metal-acid bifunctional bifunctional catalystscatalysts [8,14–16]. [8,14–16 Noble]. Noble metal-based metal-based catalysts catalysts have proven have to provenbe highly to be effective for glycerol hydrogenolysis [17]. Very few catalytic systems based on Ni [18], Ir [19], Rh highly effective for glycerol hydrogenolysis [17]. Very few catalytic systems based on Ni [18], Ir [19], [20,21], Pt [22,23], and Ru [24,25] based catalysts have so far been reported for the direct synthesis of Rh [20,21], Pt [22,23], and Ru [24,25] based catalysts have so far been reported for the direct synthesis biopropanols from glycerol, both in batch and fixed bed reactors. Previous research by Zhu et al. [26] of biopropanols from glycerol, both in batch and fixed bed reactors. Previous research by Zhu et al. [26] on liquid phase glycerol hydrogenolysis to 1-propanol and 2-propanol over Pt-H4SiW12O40/ZrO2 on liquidbifunctional phase catalysts glycerol at hydrogenolysis 200 °C and 5 MPa to H 1-propanol2 pressure revealed and 2-propanol that appropriate over Pt-Hmetal-acid4SiW 12balanceO40/ZrO 2 ◦ bifunctionalis important catalysts to promote at 200 theC anddouble 5 MPa dehydration-hydrogenation H2 pressure revealed thatability appropriate of glycerol. metal-acid In our earlier balance is importantstudy [27], to we promote have explored the double the effect dehydration-hydrogenation of different heteropolyacids abilityon the glycerol of glycerol. conversion In our and earlier studyselectivity [27], we to have propanols explored by using the effect a Pt-HPA/ZrO of different2 catalytic heteropolyacids system and ona continuous the glycerol flow, conversion fixed-bed and reactor set-up under ambient pressure. It was found that the metal dispersion and acidity of the selectivity to propanols by using a Pt-HPA/ZrO2 catalytic system and a continuous flow, fixed-bed reactorcatalyst set-up attributed under ambientto the high pressure. activity. Lin It was et al. found [28] reported that the the metal combined dispersion use of zeolite and acidity and Ni- of the catalyst attributed to the high activity. Lin et al. [28] reported the combined use of zeolite and Ni-based catalysts as a sequential two-layer catalyst system and achieved good selectivity of 1-propanol in a fixed-bed reactor. CatalystsCatalysts2018 2018, 8,, 3858, x FOR PEER REVIEW 3 of3 of18 18

based catalysts as a sequential two-layer catalyst system and achieved good selectivity of 1-propanol Despite significant research efforts, most of the reported processes were energy intensive, requiring in a fixed-bed reactor. high hydrogenDespite significant pressures and research the use efforts, of organic most of solvents the reported for the processes glycerol hydrogenolysiswere energy intensive, reaction, thusrequiring making high the hydrogen process unsustainable. pressures and Inthe addition, use of organic the conversion solvents for of the glycerol glycerol and hydrogenolysis selectivities to propanolsreaction,are thus still making not satisfied the process with these unsustainable. catalytic systems. In addition, Besides, the the conversion exact relation of glycerol between and the catalystselectivities acidity to and propanols selectivity are tostill propanols not satisfied in glycerol with these hydrogenolysis catalytic systems. needs Besides, to be properly the exact elucidated. relation Hence,between there the is a catalyst large scope acidity to improveand selectivity the sustainability to propanols of in the glycerol reaction hydrogenolysis process by making needs it to energy be efficientproperly and elucidated. increasing Hence, the process there profitability.is a large scope It to is alsoimprove most the important sustainability to find of athe highly reaction stable, process active, andby selective making it catalytic energy efficient system thatand increasing favors the the production process profitability. of propanols It byis also one most step important hydrogenolysis to find of glycerola highly under stable, ambient active, reactionand selective conditions. catalytic system that favors the production of propanols by one stepIn hydrogenolysis the present investigation, of glycerol weunder report ambient a viable reaction catalytic conditions. strategy for the direct hydrogenolysis of low-costIn glycerol the present to valued investigation, bio propanols we report over a bi-functionalviable catalytic Pd/MoO strategy3 for-Al 2theO3 directcatalysts hydrogenolysis in a fixed-bed of low-cost glycerol to valued bio propanols over bi-functional Pd/MoO3-Al2O3 catalysts in a fixed- reactor without using an organic solvent. A series of 1–4 wt% Pd/10%MoO3-Al2O3 catalysts were prepared,bed reactor carefully without characterized using an organic by different solvent. characterizationA series of 1–4 wt% techniques, Pd/10%MoO and3-Al tested2O3 catalysts in vapor were phase glycerolprepared, hydrogenolysis carefully characterized at moderate by different temperature characterization and atmospheric techniques, pressure. and tested The effectin vapor of reactionphase parametersglycerol hydrogenolysis on the catalytic at activity moderate was temperature investigated and to determineatmospheric the pressure. optimized The reactioneffect of conditions.reaction parameters on the catalytic activity was investigated to determine the optimized reaction conditions. The stability of the catalyst has been analyzed to understand the changes in the catalytic activity. The stability of the catalyst has been analyzed to understand the changes in the catalytic activity. The The research work reported herein contributes to the development of sustainable biodiesel industry, research work reported herein contributes to the development of sustainable biodiesel industry, with with the successful first use of alumina-supported palladium-molybdenum catalysts for the glycerol the successful first use of alumina-supported palladium-molybdenum catalysts for the glycerol hydrogenolysis to propanols. hydrogenolysis to propanols. 2. Results and Discussion 2. Results and Discussion 2.1. Characterization Techniques 2.1. Characterization Techniques 2.1.1. Structural Characterizations of the Catalysts 2.1.1. Structural Characterizations of the Catalysts The X-ray Diffraction (XRD) patterns of Mo-Al and Pd/Mo-Al catalysts with various palladium The X-ray Diffraction (XRD) patterns of Mo-Al and Pd/Mo-Al catalysts with various palladium contents (1–4 wt%) are shown in Figure1A, and Figure1B shows the enlarged view XRD pattern of contents (1–4 wt%) are shown in Figure 1A, and Figure 1B shows the enlarged view XRD pattern◦ of Mo-Al catalyst. For all catalysts, the diffraction peaks of γ-Al2O3 were identified at 2θ = 45.8 and Mo-Al◦ catalyst. For all catalysts, the ◦diffraction ◦peaks of γ-Al2O3 were identified at 2θ = 45.8° and 67.1 [29] while the peaks at 2θ = 23.4 and 25.7 were assigned to the crystalline MoO3 phase [30]. 67.1° [29] while the peaks at 2θ = 23.4° and 25.7° were assigned to the crystalline MoO3 phase [30]. In θ ◦ ◦ ◦ ◦ In addition, other other peaks peaks at at 2θ 2 = =20.7°, 20.7 22.0°,, 22.0 23.4°,, 23.4 and, and 25.7° 25.7 werewere identified, identified, which which are attributed are attributed to the to the Al (MoO ) phase. After the palladium addition, the characteristic diffraction peaks of MoO and Al2(MoO2 4)34 phase.3 After the palladium addition, the characteristic diffraction peaks of MoO3 and3 γ Al2Al(MoO2(MoO4)34)were3 were found found to to be be diminished. diminished. ThisThis indicatesindicates the better dispersion dispersion of of MoO MoO3 3onon γ-Al-Al2O23O in3 in ◦ amorphousamorphous form form and and saturated saturated monolayer monolayer coveragecoverage on the catalyst surface. surface. Also, Also, a apeak peak at at2θ2 θ= =33° 33 ◦ andand 59 59°[31 [31]] corresponding corresponding to to PdO PdO waswas detected,detected, which was was found found to to be be significant significant at athigher higher Pd Pd loadingsloadings and and indicates indicates fine fine dispersion dispersion at at lower lower loadings.loadings.

FigureFigure 1. 1.(A ()A X-ray) X-ray Diffraction Diffraction (XRD) (XRD) patterns patterns ofof Mo-AlMo-Al and and variousvarious Pd/Mo-Al Pd/Mo-Al catalysts catalysts (B ()B XRD) XRD patternspatterns of of Mo-Al Mo-Al catalyst catalyst in in the the enlarged enlarged view. view.

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High-temperatureHigh-temperature X-ray X-ray diffraction diffraction (HTXRD) (HTXRD) of MoO of3/Al MoO2O3 /Al support2O3 andsupport dried and 2Pd/MoO dried3- 2Pd/MoOAl2O3 catalysts3-Al2 (theO3 catalystsbest catalytic (the system best catalytic for this process) system has for been this performed process) has at various been performed temperatures at variousup to 550 temperatures °C to understand up to 550 the◦ natureC to understand of Pd interaction the nature with of the Pd interactionsupport during with thecalcination, support duringand its calcination,role in controlling and its therole catalyst in controlling activity. the As catalyst shown activity. in Figure As 2, shown Therein were Figure not2 , many There significant were not manychanges signiﬁcant in the XRD changes patterns in of the MoO XRD3/Al patterns2O3 support of MoO at different3/Al2O 3temperatures,support at different which clearly temperatures, indicate whichthe stability clearly of indicate the support the stability at calcination of the supporttemperature. at calcination The peaks temperature. appeared at The 2θ peaks= 40°, appeared46°, and 67° at ◦ ◦ ◦ 2wereθ = 40assigned, 46 , and to the 67 (hkl)were crystalline assigned tophases the (hkl) of orthorhombic crystalline phases MoO of3/Al orthorhombic2O3. However, MoO the 3HT-XRD/Al2O3. However,patterns of the the HT-XRD dried 2Pd/MoO patterns3 of-Al the2O3 dried at different 2Pd/MoO temperatures3-Al2O3 at were different found temperatures to be slightly were different found toas becompared slightly different to the as HT-XRD compared profiles to the HT-XRD of the support. proﬁles of The the support. absence The of absenceany diffraction of any diffraction patterns patternscorresponding corresponding to metallic to Pd metallic indicated Pd indicated that there that was there no formation was no formation of metallic of Pd metallic on the Pd support. on the support.Relative Relativeintensities intensities of the peak, of the as peak,well as the well peak as the width peak of width these of three these peaks, three peaks,corresponding corresponding to the tosupport, the support, showed showed slight slightchanges, changes, which whichusually usually happens happens when a when metal a ion metal like ion Pd like is part Pd isof partthe MoO of the3- MoOAl2O3 -Al crystalline2O3 crystalline framework. framework. The XRDThe XRD analysis analysis performed performed at at various various temperatures, temperatures, therefore, enabledenabled usus to to test test the the stability stability of theof catalyststhe catalysts and toand determine to determine the structure the structure of the catalysts of the catalysts at different at temperaturesdifferent temperatures varying from varying room from temperature room temperature to 550 ◦C. to 550 °C.

FigureFigure 2. 2. High-temperature X-ray diffraction (HTXRD) patterns of ( (AA)) uncalcined uncalcined Mo-Al Mo-Al and (B)) 2Pd/Mo-Al2Pd/Mo-Al catalysts. catalysts.

ScanningScanning electronelectron micrographsmicrographs (SEM)(SEM) werewere acquiredacquired toto studystudy thethe topographicaltopographical morphologymorphology ofof Mo-AlMo-Al andand 22 wt%wt% Pd/Mo-AlPd/Mo-Al catalysts. The semi-quantitative elemental analysis of the catalystscatalysts waswas performedperformed by by Scanning Scanning Electron Electron Microscopy Microscopy (SEM) coupled (SEM) coupledwith Energy with Dispersive Energy X-ray Dispersive spectroscopy X-ray (EDX).spectroscopy The SEM (EDX). images The and SEM EDX images proﬁles and of Mo-Al EDX and profiles 2Pd-Mo-Al of Mo-Al catalysts and 2Pd-Mo-Alare displayed catalysts in Figure are3. γ Asdisplayed can be seen in Figure from Figure3. As can3, the be micrographsseen from Figure reveal 3, crystallinethe micrographs aggregates reveal of crystalline MoO 3 on aggregates-Al2O3 with of a monolayer coverage. In the case of 2Pd/Mo-Al catalyst, both dense and less dense regions of bigger MoO3 on γ-Al2O3 with a monolayer coverage. In the case of 2Pd/Mo-Al catalyst, both dense and less crystallites,dense regions along of with bigger highly crystallites, dispersed along palladium with highly indicating dispersed a good palladium coverage on indicating the support a good [32], wascoverage observed. on the support [32], was observed. TheThe elementalelemental analysisanalysis datadata forfor Mo-AlMo-Al andand variousvarious loadingsloadings ofof Pd/Mo-AlPd/Mo-Al catalysts are presented in Table1. Al O was found to be the major component, with nearly similar atomic percentage in Table 1. Al22O33 was found to be the major component, with nearly similar atomic percentage of ofmolybdenum molybdenum in all in the all thecatalysts. catalysts. All Pd/Mo-Al All Pd/Mo-Al catalysts catalysts with varying with varying Pd content Pd content(1–4 wt%) (1–4 showed wt%) showed the presence of palladium, indicating the impregnation of palladium onto MoO /Al O the presence of palladium, indicating the impregnation of palladium onto MoO3/Al2O3 catalysts.3 2The3 catalysts.atomic percentages The atomic of percentages Mo and Pd from of Mo SEM-EDX and Pd from results SEM-EDX were found results to be were consistent found to with be consistentthe actual withloadings the actualused in loadings the catalyst used preparation. in the catalyst For preparation.a comparison, For the a Pd comparison, contents in the a series Pd contents of 1–4 wt% in a seriesPd/Mo-Al of 1–4 catalysts wt% Pd/Mo-Al was measured catalysts by was Inductive measured Coupled by Inductive Plasma-Atomic Coupled Plasma-Atomic Emission Spectrometer Emission Spectrometer(ICP-AES), and (ICP-AES), the results and are the presented results are in Table presented 1. in Table1.

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FigureFigure 3. 3.Scanning Scanning electron electron micrograph micrograph (SEM) (SEM) images images of of Mo-Al Mo-Al and and 2 2 wt% wt% Pd/Mo-Al Pd/Mo-Al catalysts.catalysts.

Table 1. Energy Dispersive X-ray spectroscopy (EDX) and Inductive Coupled Plasma-Atomic Emission Table 1. Energy Dispersive X-ray spectroscopy (EDX) and Inductive Coupled Plasma-Atomic Spectrometer (ICP-AES) analysis data of Mo-Al, and various Pd/Mo-Al catalysts. Emission Spectrometer (ICP-AES) analysis data of Mo-Al, and various Pd/Mo-Al catalysts. Weight Percentage (%) a ICP-AES b (wt%) Catalyst Weight Percentage (%) a ICP-AES b (wt%) Catalyst MoMo Al Al O O Pd Pd N N Mo-AlMo-Al 7.83 7.83 48.17 48.17 44.00 44.00 -- – -- – -- – 1Pd/Mo-Al1Pd/Mo-Al 6.99 6.99 43.02 43.02 48.82 48.82 0.67 0.67 0.50 0.50 0.59 0.59 2Pd/Mo-Al2Pd/Mo-Al 7.12 7.12 40.86 40.86 50.22 50.22 1.22 1.22 0.58 0.58 1.18 1.18 3Pd/Mo-Al 7.01 40.33 49.70 2.41 0.55 2.20 4Pd/Mo-Al3Pd/Mo-Al 7.24 7.01 40.99 40.33 48.21 49.70 2.41 3.07 0.55 0.49 2.20 2.97 4Pd/Mo-Al 7.24 40.99 48.21 3.07 0.49 2.97 a Determined from SEM-EDX analysis; b Metal contents determined from ICP-AES analysis. a Determined from SEM-EDX analysis; b Metal contents determined from ICP-AES analysis.

2.1.2.2.1.2. PhysicochemicalPhysicochemical PropertiesProperties of of Catalysts Catalysts

TheThe NN22 physisorption results results of of Mo-Al Mo-Al and and various various Pd/Mo-Al Pd/Mo-Al catalysts catalysts are arelisted listed in Table in Table 2. The2. 2 Thepure pure Mo-Al Mo-Al catalyst catalyst exhibited exhibited a total a total surface surface area areaof 189 of m 1892/g, m with/g, witha pore a porediameter diameter of 5.9 ofnm. 5.9 After nm. Afterincorporation incorporation of palladium of palladium into into catalysts, catalysts, the Brunauer–Emmett–Teller the Brunauer–Emmett–Teller (BET) (BET) surface surface area, area, the theaverage average pore pore volume, volume, and and the thepore pore size size of all of catalysts all catalysts were were found found to be to smaller be smaller than than the original the original Mo- Mo-AlAl catalyst. catalyst. This This change change is obvious is obvious because because of the of impregnation the impregnation of Pd of on Pd the on support the support that would that would block blockthe micropores the micropores of the of catalysts the catalysts [30]. [30].

TableTable 2. 2Physicochemical. Physicochemical properties properties of of Mo-Al Mo-Al and and various various Pd/Mo-Al Pd/Mo-Alcatalysts. catalysts. 2 3 CatalystCatalyst BETBET Surface Surface Area Area (m (m2/g)/g) PorePore Size Size (nm) (nm) PorePore Volume Volume (cm (cm3/g)/g) Mo-AlMo-Al 189 189 5.9 5.9 0.29 0.29 1Pd/Mo-Al1Pd/Mo-Al 181 181 5.8 5.8 0.28 0.28 2Pd/Mo-Al 170 5.6 0.28 2Pd/Mo-Al 170 5.6 0.28 3Pd/Mo-Al 162 5.3 0.26 3Pd/Mo-Al4Pd/Mo-Al 162 158 5.3 5.3 0.26 0.23 4Pd/Mo-Al 158 5.3 0.23

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2.1.3. Acidity Measurement by NH3–TPD Analysis

Temperature programmed desorption (TPD) experiments of γ-Al2O3, Mo-Al and 1–4 wt% Pd/Mo-Al catalysts were performed to determine the amount and strength of acid sites. The number of acid sites calculated from the desorption TPD peak area is presented in Table3. Agreeing with the literature [33], three ranges of NH3 desorption temperature should be taken into account: 50–200, 200–350, and 350–550 ◦C, which correspond to weak, moderate, and strong acid sites, respectively. ◦ The pure γ-Al2O3 catalyst exhibited weak to moderate acidity in the temperature regions of 150–350 C. The total acidity decreased with the addition of Mo, with a signiﬁcant decrease in moderate acid sites. This could be due to a decrease in surface area and blockage of acidic sites of γ-Al2O3. After incorporation of Pd in Mo-Al catalyst, weaker to moderate acid sites remained in all catalysts, with no generation of new acid sites. However, it is to be noted that there was an increase in the moderate acid site in all Pd loaded catalysts compared to Mo-Al catalyst. This indicates that weak to moderate acid centers of catalyst had a crucial role in facilitating the glycerol hydrogenolysis reaction.

Table 3. Acidities of γ-Al2O3, Mo-Al, and 1–4 wt% Pd/Mo-Al catalysts from Temperature programmed desorption of ammonia (NH3-TPD) analysis.

NH3 Uptake (µmol/g) Catalyst Total NH3 Uptake (µmol/g) Weak Moderate Strong

γ-Al2O3 298.7 426.2 – 724.9 Mo-Al 323.5 350.6 – 674.1 1Pd/Mo-Al 297.3 361.2 – 658.5 2Pd/Mo-Al 273.3 372.2 – 645.5 3Pd/Mo-Al 194.2 369.0 – 563.2 4Pd/Mo-Al 195.0 364.9 – 559.9

2.1.4. 27Al NMR Spectroscopy Solid state 27Al nuclear magnetic resonance (NMR) spectroscopy is a non-invasive and non-destructive technique vital to studying the structural modiﬁcations and coordination of aluminium nuclei within the catalyst subjected to impregnation and calcination. The chemical analysis and comparison of 27Al NMR spectra of Mo-Al and 1–4 wt% Pd/Mo-Al catalyst samples is presented in Figure4. The spectrum of Mo-Al consists of a single peak at a chemical shift of 54.1 ppm from octahedral aluminium, characteristic of as-prepared catalysts. Similar results were obtained for all the Pd/Mo-Al catalysts, indicating that palladium impregnation did not alter the basic aluminium framework. However, upon impregnation of palladium onto Mo-Al catalyst, it was observed that there was a slight change in the chemical shift of octahedral Al peak. At higher loadings, the peak was markedly broadened, and the intensity of the peak was decreased. This is probably due to the palladium interaction with the support material that causes the expulsion or distortion of aluminium atoms from the framework sites [34]. Catalysts 2018, 8, 385 7 of 18 Catalysts 2018, 8, x FOR PEER REVIEW 7 of 18

FigureFigure 4. 4.27 Al27Al nuclear nuclear magnetic magnetic resonance resonance ( (2727Al NMR) spectra spectra of of Mo-Al Mo-Al and and 1–4 1–4 wt% wt% Pd/Mo-Al Pd/Mo-Al catalysts.catalysts.

2.2.2.2. Catalytic Catalytic Studies Studies GlycerolGlycerol hydrogenolysis hydrogenolysis is is a a complex complex reactionreaction (Scheme 22)) and is generally generally believed believed to to proceed proceed viavia dehydration-hydrogenation dehydration-hydrogenation route route inin two two distinct distinct pathways, which which involve involve the the formation formation of of intermediatesintermediates (acetol (acetol and and 3-hydroxypropanaldehyde 3-hydroxypropanaldehyde (3-HPA)) (3-HPA)) by by acid-catalyzed acid-catalyzed dehydration dehydration and and subsequentsubsequent formation formation of of propanediols propanediols (1,2-PDO (1,2-PDO and and 1,3-PDO) 1,3-PDO) over over metal metal sites sites [35 [35–39].–39]. Thereafter, Thereafter, C-O hydrogenolysisC-O hydrogenolysis of 1,2-PDO of 1,2-PDOresults in results the formation in the of formation 1-propanol of (1-PrOH), 1-propanol while (1-PrOH), C-O hydrogenolysis while C-O ofhydrogenolysis 1,3-PDO gives of rise 1,3-PDO to 2-propanol gives rise to (2-PrOH). 2-propanol Another (2-PrOH). possible Another pathway possible is pathway the formation is the offormation glyceraldehyde of glyceraldehyde and 2-hydroxy and acrolein 2-hydroxy by the acrolein dehydrogenation-dehydration-hydrogenation by the dehydrogenation-dehydration- hydrogenation mechanism, in which acetol can be indirectly formed by this pathway. Direct mechanism, in which acetol can be indirectly formed by this pathway. Direct hydrogenolysis of glycerol hydrogenolysis of glycerol to propanols in one step using a single catalyst, though highly to propanols in one step using a single catalyst, though highly challenging, would be consistent and challenging, would be consistent and more preferential over a two-step process in terms of energy more preferential over a two-step process in terms of energy efﬁciency. efficiency.Catalysts 2018 , 8, x FOR PEER REVIEW 8 of 18 The present work focused on the investigation of hydrogenolysis of glycerol over highly efficient Pd/MoO3-Al2O3 catalysts with various Pd loadings (1–4 wt%) in a continuous flow, fixed-bed reactor at 210 °C under atmospheric pressure. The reaction successfully progressed to give propanols (1- PrOH + 2-PrOH) as major products. Interestingly, the presence of weak to moderates acid sites in the Pd/Mo-Al catalyst promoted the double dehydration and hydrogenation of glycerol to 1-propanol and 2-propanol, respectively. Further analysis and optimization of the reaction process helped to achieve the best glycerol conversion and selectivity to propanols. Optimisation of the reaction process included adjusting various reaction parameters such as the effect of Pd loading, reaction temperature, and hydrogen flow rate. All Pd/Mo-Al catalysts showed good activity in terms of glycerol conversion and propanol selectivity in glycerol hydrogenolysis. However, the activity differed upon altering the reaction parameters, which facilitated determining the best catalyst that resulted in the highest activity.

SchemeScheme 2. 2.Series Series and and parallel parallel networknetwork of reactions in in hydrogenolysis hydrogenolysis of of glycerol. glycerol.

2.2.1.The Effect present of Pd work Loading focused on the investigation of hydrogenolysis of glycerol over highly efficient Pd/MoO3-Al2O3 catalysts with various Pd loadings (1–4 wt%) in a continuous flow, fixed-bed reactor at 210 ◦DefiningC under atmosphericthe ideal metal pressure. loading Theof a reactioncatalyst for successfully glycerol hydrogenolysis progressed to giveis important, propanols as (1-PrOHmetal sites play a crucial role in the reaction mechanism. Therefore, the glycerol hydrogenolysis was performed using Pd/Mo-Al catalysts with varying Pd loading (1–4 wt%), and the results obtained are presented in Figure 5. In the absence of palladium over 10 wt% MoO3-Al2O3, only about 52% glycerol conversion was observed, with 76% selectivity to total propanol. The incorporation of Pd into the catalyst resulted in significant increase in the glycerol conversion and product selectivity. As can be seen from Figure 5, a maximum of 88.4% glycerol conversion with a 91.3% selectivity to total propanol was attained over 2 wt% Pd catalyst. Further, increase in Pd loading to 3 and 4 wt% showed a drop in activity, which concludes that 2 wt% Pd/Mo-Al catalyst with an appropriate number of Pd sites influenced the direct hydrogenolysis of glycerol to propanols. Thus, 2 wt% Pd catalyst has been chosen to be the optimal catalyst for further investigations.

Figure 5. Effect of Pd loading on glycerol hydrogenolysis to propanols. Reaction conditions: Reaction temperature: 210 °C, 0.1 MPa H2; H2 flow rate: 100 mL/min; 10 wt% aqueous glycerol; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanol.

Catalysts 2018, 8, x FOR PEER REVIEW 8 of 18

Catalysts 2018, 8, 385 8 of 18

+ 2-PrOH) as major products. Interestingly, the presence of weak to moderates acid sites in the Pd/Mo-Al catalyst promoted the double dehydration and hydrogenation of glycerol to 1-propanol and 2-propanol, respectively. Further analysis and optimization of the reaction process helped to achieve the best glycerol conversion and selectivity to propanols. Optimisation of the reaction process included adjusting various reaction parameters such as the effect of Pd loading, reaction temperature, and hydrogen ﬂow rate. All Pd/Mo-Al catalysts showed good activity in terms of glycerol conversion and propanol selectivity in glycerol hydrogenolysis. However, the activity differed upon altering the reaction parameters, which facilitated determining the best catalyst that resulted in the highest activity.

2.2.1. Effect of Pd Loading Scheme 2. Series and parallel network of reactions in hydrogenolysis of glycerol. Defining the ideal metal loading of a catalyst for glycerol hydrogenolysis is important, as metal sites play2.2.1. a Effect crucial of Pd role Loading in the reaction mechanism. Therefore, the glycerol hydrogenolysis was performed usingDefining Pd/Mo-Al the ideal metal catalysts loading with of a varying catalyst for Pd glycerol loading hydrogenolysis (1–4 wt%), and is important, the results as metal obtained are sites play a crucial role in the reaction mechanism. Therefore, the glycerol hydrogenolysis was presented in Figure5. In the absence of palladium over 10 wt% MoO 3-Al2O3, only about 52% glycerol performed using Pd/Mo-Al catalysts with varying Pd loading (1–4 wt%), and the results obtained are conversion was observed, with 76% selectivity to total propanol. The incorporation of Pd into the presented in Figure 5. In the absence of palladium over 10 wt% MoO3-Al2O3, only about 52% glycerol catalystconversion resulted inwas significant observed, with increase 76% selectivity in the glycerol to total conversion propanol. The and incorporation product selectivity. of Pd into the As can be seen fromcatalyst Figure resulted5, a maximum in significant of increase 88.4% glycerol in the glycerol conversion conversion with and a 91.3% product selectivity selectivity. to As total can be propanol was attainedseen from over Figure 2 wt% 5, a maximum Pd catalyst. of 88.4% Further, glycerol increase conversion in Pd with loading a 91.3% toselectivity 3 and 4to wt% total propanol showed a drop in activity,was whichattained concludes over 2 wt% thatPd catalyst. 2 wt% Further, Pd/Mo-Al increase catalyst in Pd loading with an to appropriate3 and 4 wt% showed number a drop of Pd sites influencedin activity, the direct which hydrogenolysis concludes that 2 of wt% glycerol Pd/Mo-Al to propanols. catalyst with Thus, an appropriate 2 wt% Pd number catalyst of has Pd beensites chosen influenced the direct hydrogenolysis of glycerol to propanols. Thus, 2 wt% Pd catalyst has been to be the optimal catalyst for further investigations. chosen to be the optimal catalyst for further investigations.

Figure 5.FigureEffect 5. Effect of Pd of loading Pd loading on on glycerol glycerol hydrogenolysis hydrogenolysis to to propanols. propanols. Reaction Reaction conditions: conditions: Reaction Reaction temperature:◦ 210 °C, 0.1 MPa H2; H2 flow rate: 100 mL/min; 10 wt% aqueous glycerol; 0.5 g catalyst; temperature: 210 C, 0.1 MPa H2;H2 ﬂow rate: 100 mL/min; 10 wt% aqueous glycerol; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanol. Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanol.

2.2.2. Effect of Reaction Temperature To assess the inﬂuence of reaction temperature on glycerol conversion and propanol selectivity, the glycerol hydrogenolysis reaction was performed over 2 wt% Pd/Mo-Al catalyst at different reaction temperatures going from 170 to 250 ◦C, and the results are shown in Figure6. It is obvious that the reaction temperature had a positive inﬂuence on the glycerol conversion, as suggested by previous studies [40,41]. With the increase in the temperature from 170 ◦C to 230 ◦C, the glycerol conversion increased steadily from 70.6% up to 92.6%. The highest total propanol selectivity of 91.3% was obtained at a reaction temperature of 210 ◦C. Further increase of temperature to 250 ◦C caused a decrease in glycerol conversion and 1-PrOH selectivity. However, it is noteworthy that the selectivity to 2-PrOH elevated at a higher temperature. Therefore, the results suggest that a reaction temperature of 210 ◦C promoted excessive C-O hydrogenolysis of glycerol to produce the highest amounts of 1-propanol and Catalysts 2018, 8, x FOR PEER REVIEW 9 of 18

2.2.2. Effect of Reaction Temperature To assess the influence of reaction temperature on glycerol conversion and propanol selectivity, the glycerol hydrogenolysis reaction was performed over 2 wt% Pd/Mo-Al catalyst at different reaction temperatures going from 170 to 250 °C, and the results are shown in Figure 6. It is obvious that the reaction temperature had a positive influence on the glycerol conversion, as suggested by previous studies [40,41]. With the increase in the temperature from 170 °C to 230 °C, the glycerol conversion increased steadily from 70.6% up to 92.6%. The highest total propanol selectivity of 91.3% Catalysts 2018was, 8, 385obtained at a reaction temperature of 210 °C. Further increase of temperature to 250 °C caused a 9 of 18 decrease in glycerol conversion and 1-PrOH selectivity. However, it is noteworthy that the selectivity to 2-PrOH elevated at a higher temperature. Therefore, the results suggest that a reaction temperature 2-propanol,of in210 addition °C promoted to conﬁrmingexcessive C-O that hydrogenolysis reaction temperature of glycerol to produce had a signiﬁcantthe highest amounts effect on of the1- product propanol and 2-propanol, in addition to confirming that reaction temperature had a significant effect distributionon ofthe 1-PrOHproduct distribution and 2-PrOH. of 1-PrOH and 2-PrOH.

Figure 6. FigureEffect 6. ofEffect Reaction of Reaction temperature temperature on on glycerol glycerol hydrogenolysis hydrogenolysis to propanols. to propanols. Reaction Reaction conditions: Reaction temperature = 170–250 °C, Reduction temperature = 350 °C; 0.1 MPa H2; H2 flow conditions: Reaction temperature = 170–250 ◦C, Reduction temperature = 350 ◦C; 0.1 MPa H ;H flow rate = 100 mL/min; 10 wt% aqueous glycerol; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2 2 rate = 1002-PrOH: mL/min 2-Propanol;; 10 wt% Total aqueous PrOHs: glycerol; Total propanols. 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanols. 2.2.3. Effect of Hydrogen Flow Rate 2.2.3. Effect ofHydrogen Hydrogen is a Flow key reactant Rate in hydrogenolysis reaction, and it is important to understand the Hydrogeninfluence is of a the key hydrogen reactant flow in rate hydrogenolysis on glycerol hydrogenolysis. reaction, andOptimising it is important the hydrogen to flow understand rate the will add value to the process and will balance the higher costs of high-pressure apparatus. Hence the influence ofreaction the hydrogen was investigated flow by rate varying on glycerol the hydrogen hydrogenolysis. pressures at 60, 80, Optimising 100, and 120 the mL/min hydrogen over 2 flow rate will add valuewt% Pd/Mo-Al to the process catalyst andat 210 will °C under balance atmospheric the higher pressure. costs As ofcan high-pressure be observed from apparatus. Figure 7, the Hence the reaction wasglycerol investigated conversion and by selectivity varying of the 1-PrOH hydrogen increased pressures with an increase at 60, in 80, hydrogen 100, and flow 120 rate mL/minfrom over 2 wt% Pd/Mo-Al60 to 120 mL/min. catalyst However, at 210 ◦ Cthe under total PrOH atmospheric selectivity was pressure. found to As be canthe highest be observed at a hydrogen from Figure7, flow rate of 100 mL/min. While when the hydrogen flow rate was 120 mL/min, total PrOH selectivity the glycerolslightly conversion dropped, and in spite selectivity of distinct of increment 1-PrOH in increased glycerol conversion, with an increase to 94%. This in hydrogen is probably flow rate from 60 tobecause 120 mL/min. at higher However,hydrogen flow the rates total excessive PrOH selectivityhydrogenolysis was occurs, found leading to be to the the highest formation at of a hydrogen flow rate ofdegradative 100 mL/min. products. While A similar when observation the hydrogen was made flow in rate previous was studies 120 mL/min, [42]. Thus, total 100 mL/min PrOH selectivity slightly dropped,hydrogen in flow spite rate of was distinct considered increment to be optimum in glycerol for this conversion, reaction, resulting to 94%. in Thisa maximum is probably total because PrOHs selectivity of 91.3% at 88.4% glycerol conversion. at higher hydrogen flow rates excessive hydrogenolysis occurs, leading to the formation of degradative products. A similar observation was made in previous studies [42]. Thus, 100 mL/min hydrogen flow rate was considered to be optimum for this reaction, resulting in a maximum total PrOHs selectivity of 91.3% at 88.4% glycerol conversion. Catalysts 2018, 8, x FOR PEER REVIEW 10 of 18

Figure 7.FigureEffect 7. Effect of Hydrogen of Hydrogen ﬂow flow rate rate on on glycerol glycerol hydrogenolysis to propanols. to propanols. Reaction Reaction conditions: conditions: Reaction temperature = ◦210 °C; H2 flow rate = 60, 80, 100 & 120 mL/min, 0.1 MPa H2; 10 wt% aqueous Reaction temperature = 210 C; H2 ﬂow rate = 60, 80, 100 & 120 mL/min, 0.1 MPa H2; 10 wt% aqueous glycerol; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: glycerol; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanols. Total propanols. 2.2.4. Effect of Glycerol Concentration The glycerol concentration serves as one of the most important parameters which has a significant effect on the activity and selectivity of glycerol hydrogenolysis reaction. It is known that a low concentration of glycerol is favorable to increase the glycerol conversion [17,43]. Figure 8 shows the activity results at different glycerol concentrations (5–20 wt%), which was performed over 2 wt% Pd/Mo-Al catalyst to identify the best activity. From the results, it was observed that by increasing the glycerol concentration from 5 wt% to 20 wt%, both the glycerol conversion and selectivity to total propanols declined. As the glycerol content increased, the glycerol conversion and the selectivity towards propanols decreased considerably. With 5 wt% glycerol feed, 88% of glycerol converted, with about 80% selectivity to total propanols. At 10 wt% glycerol feed, the selectivity to total propanols was improved to 91.3% at nearly the same glycerol conversion, indicating the excessive hydrogenolysis of glycerol to propanols. At higher glycerol concentration (15 and 20 wt%), it was observed that both conversion and selectivity dropped considerably, which is ascribed to the fact that the reaction rate is lowered due to high viscosity of the glycerol feed and the imbalance between the catalytically active sites and the excess glycerol available to react at higher concentrations. Consequently, 10 wt% glycerol feed was considered the optimal concentration to attain the highest glycerol conversion of glycerol and selectivity to total propanols.

Figure 8. Effect of glycerol concentration on glycerol hydrogenolysis to propanols. Reaction conditions: Reaction temperature = 210 °C; H2 flow rate = 100 mL/min, 0.1 MPa H2; 5–10 wt% glycerol

Catalysts 2018, 8, x FOR PEER REVIEW 10 of 18

Catalysts 2018Figure, 8, 385 7. Effect of Hydrogen flow rate on glycerol hydrogenolysis to propanols. Reaction conditions: 10 of 18 Reaction temperature = 210 °C; H2 flow rate = 60, 80, 100 & 120 mL/min, 0.1 MPa H2; 10 wt% aqueous glycerol; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: 2.2.4. EffectTotal of propanols. Glycerol Concentration The glycerol concentration serves as one of the most important parameters which has a signiﬁcant 2.2.4. Effect of Glycerol Concentration effect on the activity and selectivity of glycerol hydrogenolysis reaction. It is known that a low concentrationThe glycerol of glycerol concentration is favorable serves to increase as one the of the glycerol most conversion important parameters [17,43]. Figure which8 shows has a the activitysignificant results effect at different on the activity glycerol and concentrations selectivity of glycerol (5–20 hydrogenolysis wt%), which wasreaction. performed It is known over that 2 wt% Pd/Mo-Ala low concentration catalyst to identify of glycerol the is bestfavorable activity. to increase From the glycerol results, conversion it was observed [17,43]. Figure that by 8 shows increasing the activity results at different glycerol concentrations (5–20 wt%), which was performed over 2 wt% the glycerol concentration from 5 wt% to 20 wt%, both the glycerol conversion and selectivity to total Pd/Mo-Al catalyst to identify the best activity. From the results, it was observed that by increasing propanolsthe glycerol declined. concentration As the glycerolfrom 5 wt% content to 20 wt%, increased, both the theglycerol glycerol conversion conversion and selectivity and the to selectivity total towardspropanols propanols declined. decreased As the glycerol considerably. content Withincreased, 5 wt% the glycerol glycerol feed,conversion 88% ofand glycerol the selectivity converted, with abouttowards 80% propanols selectivity decreased to total considerably. propanols. At With 10 wt%5 wt% glycerol glycerol feed, feed, the 88% selectivity of glycerol to totalconverted, propanols was improvedwith about to 80% 91.3% selectivity at nearly to the total same propanols. glycerol Atconversion, 10 wt% glycerol indicating feed, the the excessive selectivity hydrogenolysis to total of glycerolpropanols to propanols. was improved At higherto 91.3% glycerol at nearly concentration the same glycerol (15 and conversion, 20 wt%), indicating it was observed the excessive that both conversionhydrogenolysis and selectivity of glycerol dropped to propanols. considerably, At higher which glycerol is ascribed concentration to the fact(15 and that 20 the wt%), reaction it was rate is loweredobserved due tothat high both viscosity conversion of and the glycerolselectivity feed dropped and considerably, the imbalance which between is ascribed the catalytically to the fact that active sites andthe reaction the excess rateglycerol is lowered available due to high to react viscosity at higher of the concentrations.glycerol feed and Consequently, the imbalance between 10 wt% glycerolthe catalytically active sites and the excess glycerol available to react at higher concentrations. feed was considered the optimal concentration to attain the highest glycerol conversion of glycerol Consequently, 10 wt% glycerol feed was considered the optimal concentration to attain the highest and selectivityglycerol conversion to total propanols.of glycerol and selectivity to total propanols.

FigureFigure 8. Effect 8. Effect of glycerol of glycerol concentration concentration on glycerol on glycerol hydrogenolysis hydrogenolysis to propanols. to propanols. Reaction Reaction conditions: conditions: Reaction temperature◦ = 210 °C; H2 flow rate = 100 mL/min, 0.1 MPa H2; 5–10 wt% glycerol Reaction temperature = 210 C; H2 ﬂow rate = 100 mL/min, 0.1 MPa H2; 5–10 wt% glycerol feed; 0.5 g

catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanols.

2.2.5. Effect of the Partial Pressure of Glycerol Figure9 shows the effect of partial pressure of glycerol on glycerol conversion, and total propanols ◦ selectivity for the 2 Pt/Mo-Al catalyst at 210 C, 100 mL/min H2 flow rate, 10 wt% glycerol solution, and at different glycerol feed flow rates of 0.5, 1.0, 1.5, and 2.0 mL/h. With the increase in partial pressure of glycerol from 8.3–13.0 mm Hg, the conversion of glycerol decreased from 88.4% to 71%, and the selectivity to total propanols decreased almost two-fold. This decrease in conversion and selectivity can be attributed to a decrease in the number of active sites due to the increase in the flow of glycerol feed [44]. Catalysts 2018, 8, x FOR PEER REVIEW 11 of 18

feed; 0.5 g catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanols.

2.2.5. Effect of the Partial Pressure of Glycerol Figure 9 shows the effect of partial pressure of glycerol on glycerol conversion, and total propanols selectivity for the 2 Pt/Mo-Al catalyst at 210 °C, 100 mL/min H2 flow rate, 10 wt% glycerol solution, and at different glycerol feed flow rates of 0.5, 1.0, 1.5, and 2.0 mL/h. With the increase in partial pressure of glycerol from 8.3–13.0 mm Hg, the conversion of glycerol decreased from 88.4% to 71%, and the selectivity to total propanols decreased almost two-fold. This decrease in conversion Catalystsand2018 selectivity, 8, 385 can be attributed to a decrease in the number of active sites due to the increase in the11 of 18 flow of glycerol feed [44].

FigureFigure 9. Effect 9. Effect of the of the partial partial pressure pressure of of glycerol glycerol onon glycerolglycerol hydrogenolysis hydrogenolysis to topropanols. propanols. Reaction Reaction ◦ conditions:conditions: Reaction Reaction temperature temperature = 210= 210C; °C; H H2 2ﬂow flow raterate = 100 100 mL/min, mL/min, 0.1 0.1 MPa MPa H2; H102 ;wt% 10 wt% aqueous aqueous glycerol;glycerol; 0.5, 1.0,0.5, 1.51.0, and1.5 and 2.0 2.0 mL/h mL/h glycerol glycerol feed;feed; 0.5 g catalyst; catalyst; Reaction Reaction time: time: 6 h; 6 1-PrOH: h; 1-PrOH: 1-propanol; 1-propanol; 2-PrOH:2-PrOH: 2-Propanol; 2-Propanol; Total Total PrOHs: PrOHs: Total Total propanols. propanols.

2.2.6.2 Effect.2.6. Effect of Contact of Contact Time Time (W/F) (W/F) The dependenceThe dependence of theof the rate rate of of glycerol glycerol hydrogenolysis hydrogenolysis on on contact contact time time (W/F) (W/F) in the in therange range of 0.4 of 0.4 to 2.0to g 2.0 mL g− 1mLh−1 at h 210 at 210◦C, °C, 0.5 0.5 mL/h mL/h 10 10 wt% wt% glycerolglycerol feed was was studied studied over over 2Pd/Mo-Al 2Pd/Mo-Al catalyst catalyst by by varyingvarying the weight the weight of the of the catalyst. catalyst. As As can can be be notednoted from Figure Figure 10, 10 the, the glycerol glycerol conversion conversion increases increases steadily from 60% to 88.4% with an increase in the contact time. Similarly, the selectivity to total steadily from 60% to 88.4% with an increase in the contact time. Similarly, the selectivity to total propanols was also found to increase substantially from 68% to 91.3% upon increasing the contact propanolstime. This was observation also found suggests to increase that greater substantially contact time from enables 68%to the 91.3% strong upon adsorption increasing of glycerol the contacton time.to This the observationcatalytically active suggests sites that and greaterfacilitates contact the excessive time enables hydrogenolysis the strong reaction adsorption of glycerol of glycerol to on toproduce the catalytically double dehydration-dehydrogenation active sites and facilitates the products excessive 1-propanol hydrogenolysis and 2-propanol, reaction which of glycerol is in to producegood double agreement dehydration-dehydrogenation with the previous studies [27]. products 1-propanol and 2-propanol, which is in good agreement with the previous studies [27]. Catalysts 2018, 8, x FOR PEER REVIEW 12 of 18

FigureFigure 10. Effect 10. Effect of contact of contact time on time glycerol on glycerol hydrogenolysis hydrogenolysis to propanols. to propanols. Reaction Reaction conditions: conditions: Reaction Reaction temperature◦ = 210 °C; H2 flow rate = 100 mL/min, 0.1 MPa H2; 10 wt% aqueous glycerol; 0.5 temperature = 210 C; H2 flow rate = 100 mL/min, 0.1 MPa H2; 10 wt% aqueous glycerol; 0.5 mL/h glycerolmL/h feed; glycerol 0.2–0.5 feed; mg 0.2–0.5 catalyst; mg catalyst; Reaction Reaction time: 6time: h; 1-PrOH: 6 h; 1-PrOH: 1-propanol; 1-propanol; 2-PrOH: 2-PrOH: 2-Propanol; 2-Propanol; Total Total PrOHs: Total propanols. PrOHs: Total propanols. From all the above sequence of experiments, the optimal set of reaction conditions to accomplish From all the above sequence of experiments, the optimal set of reaction conditions to accomplish the best possible glycerol conversion and total propanols selectivity over 2Pd/Mo-Al catalyst were the best possible glycerol conversion and total propanols selectivity over 2Pd/Mo-Al catalyst were identified as 210 °C, 100 mL/min H2 flow rate, 10 wt% glycerol concentration, 8.3 mm Hg partial ◦ identifiedpressure as 210of glycerol,C, 100 and mL/min at 1.0 g HmL2 flow−1 h contact rate, 10time. wt% Under glycerol these concentration,best set of conditions, 8.3 mm the Hg most partial −1 pressureactive of 2Pd/Mo-Al glycerol, andcatalysts at 1.0 was g mLtested forh contact stabilitytime. and reusability, Under these and bestthe results set of are conditions, analyzed. the most active 2Pd/Mo-Al catalysts was tested for stability and reusability, and the results are analyzed. 2.2.7. Time on Stream Experiment Time on stream experiment for 8 h was conducted over the best catalyst 2 wt% Pd/Mo-Al under the similar reaction conditions to investigate the stability and activity of catalyst (Figure 11). The conversion of glycerol progressively increased from 82% and reached a maximum of 88.4% at 3 h and remained constant up until 6 h. Also, the selectivity to total PrOH also maximized at 3 h, remained the same until 5 h, and presented a slight decline during 6–8 h. This decline in activity might be probably due to the fact that the catalyst suffers slow deactivation during the reaction due to carbon deposition. A similar finding was reported in our previous studies [8,35].

Figure 11. Time-on-stream experiment over fresh 2 wt% Pd/Mo-Al2O3 catalyst. Reaction conditions: Reaction temperature = 210 °C; 0.1 MPa H2; H2 Flow rate = 100 mL/min, 10 wt% aqueous glycerol; 0.5 g catalyst; Total PrOHs: Total propanols.

Catalysts 2018, 8, x FOR PEER REVIEW 12 of 18

Figure 10. Effect of contact time on glycerol hydrogenolysis to propanols. Reaction conditions: Reaction temperature = 210 °C; H2 flow rate = 100 mL/min, 0.1 MPa H2; 10 wt% aqueous glycerol; 0.5 mL/h glycerol feed; 0.2–0.5 mg catalyst; Reaction time: 6 h; 1-PrOH: 1-propanol; 2-PrOH: 2-Propanol; Total PrOHs: Total propanols.

From all the above sequence of experiments, the optimal set of reaction conditions to accomplish the best possible glycerol conversion and total propanols selectivity over 2Pd/Mo-Al catalyst were identified as 210 °C, 100 mL/min H2 flow rate, 10 wt% glycerol concentration, 8.3 mm Hg partial pressureCatalysts 2018 of, 8glycerol,, 385 and at 1.0 g mL−1 h contact time. Under these best set of conditions, the12 most of 18 active 2Pd/Mo-Al catalysts was tested for stability and reusability, and the results are analyzed.

2.2.7. Time on Stream Experiment Time on on stream stream experiment experiment for for 8 h 8 was h was conducted conducted over over the best the bestcatalyst catalyst 2 wt% 2 Pd/Mo-Al wt% Pd/Mo-Al under theunder similar the similar reaction reaction conditions conditions to investigate to investigate the stability the stability and activity and activity of catalyst of catalyst (Figure (Figure 11). The11). conversionThe conversion of glycerol of glycerol progressively progressively increased increased from 82% from and 82% reached and reached a maximum a maximum of 88.4% of at 88.4% 3 h and at remained3 h and remained constant constantup until 6 up h.until Also, 6 the h. Also,selectivity the selectivity to total PrOH to total also PrOH maximized also maximized at 3 h, remained at 3 h, theremained same theuntil same 5 h, until and 5presented h, and presented a slight a decline slight decline during during 6–8 h. 6–8 This h. Thisdecline decline in activity in activity might might be probablybe probably due due to tothe the fact fact that that the the catalyst catalyst suffers suffers slow slow deactivation deactivation during during the the reaction reaction due due to to carbon deposition. A similar finding ﬁnding was reported in our previous studies [[8,35].8,35].

Figure 11. Time-on-stream experimentexperiment overover freshfresh 22 wt%wt%Pd/Mo-Al Pd/Mo-Al2O33 catalyst.catalyst. Reaction Reaction conditions: conditions: ◦ Reaction temperature == 210 °C;C; 0.1 MPa HH22;H; H2 FlowFlow rate = 100 100 mL/min, 10 10 wt% wt% aqueous aqueous glycerol; glycerol; 0.5 g catalyst; Total PrOHs: Total propanols.propanols.

2.3. Reusability of the Catalyst

To further understand the stability and reusability of catalyst, the spent 2Pd/Mo-Al catalyst was regenerated by activating the catalyst in air ﬂow (100 mL/min) at 500 ◦C for 2 h and then tested in glycerol hydrogenolysis time-on-stream experiment under the similar reaction conditions used for the fresh catalyst. The performance of 2Pd/Mo-Al catalyst upon re-use is illustrated in Figure 12A. The results were reproduced over the used catalyst and demonstrated a similar pattern to that of the fresh catalyst during 1–6 h. However, there is a slight decline in glycerol conversion and a signiﬁcant drop in the selectivity to total propanols at later hours. The cause for the decrease in activity might be due to the catalyst deactivation over time due to agglomeration or carbon deposition. The used and reactivated catalysts were further analyzed by various characterization techniques, and the results are presented in Table4 & Figure 12.

Table 4. Studies on the used 2Pd/Mo-Al catalyst.

Conversion Selectivity of BET Surface Area Total Acidity Catalyst 2 (%) Total PrOHs (%) (m /g) (NH3 µmol/g) Fresh 88.4 91.3 170 645.5 Used 87.0 90.0 162 594.0 Reactivated 88.0 90.9 168 617.2 Catalysts 2018, 8, 385 13 of 18 Catalysts 2018, 8, x FOR PEER REVIEW 14 of 18

FigureFigure 12. (A) Time-on-stream experimentexperiment overover usedused 22 wt%wt% Pd/Mo-AlPd/Mo-Al catalyst; catalyst; ( B) thermogravimetric analysisanalysis (TGA)(TGA) proﬁleprofile ofof freshfresh andand usedused catalyst;catalyst; ((CC)) XRD XRD pattern pattern of of fresh, fresh, used used and and reactivated reactivated catalyst;catalyst; ((DD)) SEMSEM imageimage ofof usedused 2Pd/Mo-Al2Pd/Mo-Al catalyst. catalyst.

3. MaterialsThe XRD and pattern, Methods SEM image, BET SA, and total acidity from NH3-TPD analysis of the used 2Pd/Mo-Al catalyst showed no significant changes, indicating that the catalytic structure remained intact3.1. Catalyst during Preparation the course of the reaction. Thermogravimetric analysis (TGA) was used to characterize the used catalyst to understand the mode of deactivation. The TGA measurements were performed A series of palladium catalysts on 10 wt% MoO3/Al2O3 with varying palladium loading from over a temperature range of 50 ◦C to 750 ◦C at a heating rate of 10 ◦C/min under a constant flow of 1.0–4.0 wt% was prepared by wet impregnation method. Tetraammine palladium (II) nitrate solution nitrogen (20 mL/min). The recorded TG curves of fresh and used catalysts presented as Figure 12B (10 wt% in H2O, produced by Sigma Aldrich Co., Ltd., St. Louis, MO, USA) was used as a precursor consist of four mass loss processes common in both fresh and used catalysts and an additional mass on the support. 10 wt% MoO3/Al2O3 (2–5 mm pellets) was obtained from Riogen. The prepared loss process observed in used catalyst. The first step (I) between 100 and 130 ◦C corresponds to removal catalysts were dried overnight at 100 °C and subsequently calcined at 500 °C for 2 h in air. The of physisorbed water; the second mass loss region (II) around 250 ◦C indicates the loss of coordinated prepared catalysts were labelled as xPd/Mo-Al, where x refers to Pd loading, Mo refers to MoO3, and water molecules; the fourth weight loss step (IV) between 450–500 ◦C is attributed to the removal of Al refers to Al2O3. strongly bound hydroxyl groups from alumina matrix; and a small weight loss (V) above 700 ◦C is due to the sublimation of molybdena [45]. In addition, a large weight loss region (III) extended above 3.2. Catalyst Characterization 380–500 ◦C in the used catalyst corresponds to the loss of carbonaceous materials [46], indicating the deactivationThe X-ray Diffraction of the catalyst (XRD) by of carbon catalysts deposition. was performed The total on weight a Miniflex loss due X-ray to heatingdiffractometer at the temperature(Rigaku, Neu-Isenburg, range 50–750 Germany)◦C for the with used Ni-filtered catalyst was Cu-Kα found radiation to be 4.0 wt% (λ = while 1.392 it Å). was The just angles 2.2 wt% of forscanning the fresh were catalyst, from 2° which to 90° clearly with a indicatesrate of 2°/min, that the with excess the beam weight voltage loss is and due a to beam the deposition current of of30 carbonkV and species 15 mA, and respectively. undesired High materials Temperature on the catalyst X-ray surfacediffraction during (HTXRD) the reaction. profiles of the catalysts as a function of temperature were carried out using Bruker D8 Advance X-ray diffractometer equipped with the Anton-Parr heating accessory. Nitrogen physisorption analysis of the catalysts was carried at −196 °C under liquid N2 with a Quantachrome Autosorb 1 instrument. As a pretreatment of the N2 physisorption analysis, each catalyst sample is degassed under vacuum for 6 h at 250 °C. The multi-point Brunauer–Emmet–Teller

Catalysts 2018, 8, 385 14 of 18

3. Materials and Methods

3.1. Catalyst Preparation

A series of palladium catalysts on 10 wt% MoO3/Al2O3 with varying palladium loading from 1.0–4.0 wt% was prepared by wet impregnation method. Tetraammine palladium (II) nitrate solution (10 wt% in H2O, produced by Sigma Aldrich Co., Ltd., St. Louis, MO, USA) was used as a precursor on the support. 10 wt% MoO3/Al2O3 (2–5 mm pellets) was obtained from Riogen. The prepared catalysts were dried overnight at 100 ◦C and subsequently calcined at 500 ◦C for 2 h in air. The prepared catalysts were labelled as xPd/Mo-Al, where x refers to Pd loading, Mo refers to MoO3, and Al refers to Al2O3.

3.2. Catalyst Characterization The X-ray Diffraction (XRD) of catalysts was performed on a Miniflex X-ray diffractometer (Rigaku, Neu-Isenburg, Germany) with Ni-filtered Cu-Kα radiation (λ = 1.392 Å). The angles of scanning were from 2◦ to 90◦ with a rate of 2◦/min, with the beam voltage and a beam current of 30 kV and 15 mA, respectively. High Temperature X-ray diffraction (HTXRD) profiles of the catalysts as a function of temperature were carried out using Bruker D8 Advance X-ray diffractometer equipped with the Anton-Parr heating accessory. ◦ Nitrogen physisorption analysis of the catalysts was carried at −196 C under liquid N2 with a Quantachrome Autosorb 1 instrument. As a pretreatment of the N2 physisorption analysis, each catalyst sample is degassed under vacuum for 6 h at 250 ◦C. The multi-point Brunauer–Emmet–Teller (BET) method was used to calculate the specific surface areas of each catalyst and the Barrett–Joyner–Halenda model (BJH) method was used to measure the average pore diameter and pore volumes. Scanning electron microscopy (SEM) was used to study the morphology of the catalyst samples, and the elemental identification of the catalysts was performed by Energy Dispersive X-Ray Analyzer coupled to Phenom XL scanning electron microscopy (SEM-EDX). Before analysis by SEM, each sample was mounted on an aluminum support using double adhesive carbon tape. At 10 kV beam voltage and 5000× magnification, the micrographs of the catalysts were captured using a backscatter electron detector (BSD). The elemental analysis was performed at high resolution (15 kV of beam voltage) and high vacuum pressure (1 Pa) with a secondary electron detector (SED), where the point and mapping analysis for element identification was performed using a Phenom Pro Suite software. The amount of Pd in all Pd/Mo-Al catalysts was quantitatively analyzed by Inductively coupled plasma atomic emission spectrometry (Agilent Technologies-4200MP-AES, Santa Clara, CA, USA). The samples were prepared by acid digestion of catalyst (~10 mg in 2 mL aquaregia) at 60 ◦C followed by dilution to desired concentration. Temperature programmed desorption of ammonia (NH3-TPD) experiments were conducted on AutoChem 2910 (Micromeritics, Norcross, GA, USA) instrument. In a typical experiment, 100 mg of oven-dried sample was pretreated by passage of high purity (99.995%) helium (50 mL min−1) at 200 ◦C for one hour. After pretreatment, the sample was saturated with highly pure anhydrous ammonia −1 ◦ (50 mL min ) with a mixture of 10% NH3–He at 80 C for 1 h and subsequently flushed with He flow (50 mL min−1) at 80 ◦C for 30 min to remove physisorbed ammonia. TPD analysis was carried out ◦ ◦ −1 from ambient temperature to 700 C at a heating rate of 10 C min . The amount of NH3 desorbed was calculated using the GRAMS/32 software. Solid state 27Al Nuclear Magnetic Resonance Spectroscopy (27Al NMR) technique was used to identify the basic aluminium framework structure of catalysts on DD2 Oxford Magnet AS-500MHz spectrometer (Agilent Technologies) using aluminium oxide (Aldrich) as a probe. The chemical shifts (δ) are revealed in ppm. Thermo gravimetric analysis (TGA) was performed using a Perkin Elmer TGA-7 from 35 ◦C to 700 ◦C with a heating rate of 10 ◦C per minute under the flow of nitrogen. Catalysts 2018, 8, 385 15 of 18

3.3. Catalyst Testing The glycerol hydrogenolysis experiments were conducted in the vapour phase under atmospheric pressure in a vertical fixed bed quartz reactor (40 cm length, 9 mm i.d.) using 0.5 g of catalyst. Before ◦ −1 the reaction, the catalysts were pretreated at 350 C for 2 h in flowing H2 (60 mL min ). After cooling down to the reaction temperature (210 ◦C), an aqueous solution of 10 wt% glycerol was introduced into the reactor using a feed pump along with the flow of hydrogen (100 mL/min). The reaction products were condensed in an ice–water trap and collected hourly for analysis on a gas chromatograph GC-2014 (Shimadzu, Kyoto, Japan) equipped with a DB-wax 123-7033 (Agilent) capillary column (0.32 mm i.d., 30 m long) and flame ionization detector. The conversion of glycerol (1) and selectivity (2) of products were calculated as follows: moles of glycerol (in) − moles of glycerol (out) Conversion (%) = × 100 moles of glycerol (in)

moles of one product Selectivity (%) = × 100 moles of all products

4. Conclusions A simple and highly efficient protocol of direct hydrogenolysis of glycerol to 1-propanol and 2-propanol over a series of 1–4 wt% Pd/MoO3-Al2O3 catalysts has been demonstrated. Pd/Mo-Al catalysts were prepared by wetness impregnation method and characterized by XRD, SEM, NH3-TPD, 27Al NMR, and BET to evaluate the physical and chemical properties of catalysts. The hydrogenolysis of glycerol was performed in vapour phase under atmospheric pressure. Among the catalysts tested, 2 wt% Pd/Mo-Al catalyst established high activity and selectivity in the glycerol hydrogenolysis, with 91.3% total propanol selectivity at 88.4% glycerol conversion at best reaction conditions of ◦ 210 C, 1 bar pressure, 100 mL/min H2 flow rate, 10 wt% aq. Glycerol, and 8.3 mm Hg partial pressure of glycerol. The weak to moderate acidity of the catalyst was found to affect the degree of glycerol dehydroxylation, and promoted double dehydration of glycerol to form 1-propanol and 2-propanol. A comprehensive study on reaction parameters revealed the optimized reaction conditions and it was found that the reaction temperature, partial pressure of glycerol, and contact time had a substantial influence on the glycerol conversion and propanol selectivity. Further, the time on stream studies demonstrated that the catalyst remained stable for a longer period without great loss in the conversion and selectivity to products. The activity results were well correlated with the results obtained from catalyst characterization. The study signifies a sustainable technology that can not only develop the opportunity of biodiesel industry, but also deliver a green chemical production route from biomass-derived glycerol.

Author Contributions: Conceptualization, Methodology, Formal Analysis, Investigation, Writing-Original Draft Preparation, S.S.P.; Supervision, and Draft Review, S.B. Funding: This research received no external funding. Acknowledgments: S.S.P. would like to acknowledge Industry Connect Seed Fund support from the Monash Energy Materials and Systems Institute and Discovery Seed Fund Scheme from Monash Engineering. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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